嵌套在泛型中扩展

Question

I have three classes:

class ClassR {}
class ClassA<T extends ClassR>{}    
class ClassB<E extends ClassA<T extends ClassR>> extends ClassA<T> {
    void foo(T param) {
    }

    void bar(E param) {
    }
}

The third class does not compile unless I change it to

class ClassB<E extends ClassA<T>, T extends ClassR> extends ClassA<T> {
    void foo(T bar) {
    }

    void bar(E param) {
    }
}

Is there a way to keep only E parameter I need to pass when creating ClassB, and T being inferred? For example, it would be convenient to use:

new ClassB<ClassA<ClassR>>()

instead of:

new ClassB<ClassA<ClassR>, ClassR>()

Answer 1

This even simpler approach might work for you:

class ClassR {}
class ClassA<T extends ClassR>{}    

class ClassB<T extends ClassR> extends ClassA<T> {
    void foo(T bar) {
    }

    void bar(ClassA<T> param) {
    }
}

And usage then bypasses any reference to ClassA to become:

class SubR extends ClassR {}

ClassB<SubR> obj = new ClassB<SubR>();

Answer 2

Not sure if this is the answer you want but surely the simplest version reads:

class ClassR {
}

class ClassA<T extends ClassR> {
}

class ClassB<T extends ClassR> extends ClassA<T> {

    void foo(T bar) {
    }
}

public void test() {
    ClassB<ClassR> classB = new ClassB<>();
}

Answer 3

As type E extends ClassA you can safely omit the ClassA type parameter in its declaration.
Proper ClassA type parameter is enforced in the second parameter to ClassB .
See below code for illustration :

class ClassB<E extends ClassA, T extends ClassR> extends ClassA<T> {

  private ClassA ob;

  public ClassB(E e, T t)  {
    super(t);
    ob = e;
  }

}

Usage sample:

class ClassR { 
   public ClassR() {};
}

class ClassS extends ClassR {
  private int x;
  public ClassS(int x) {
     super();
     this.x = x;
  }
}

public static void test() {
  ClassS data1 = new ClassS(1);
  ClassB <ClassB, ClassS> first = new ClassB<>(null, data1);
  ClassS data2 = new ClassS(2);
  ClassB <ClassB, ClassS> second = new ClassB<>(first, data2);
}